In the agriculture, in particular, the cultivation of a rice plant, there is a case of using a leaf color scale, that is, a color chart of a leaf color density which becomes an index of fertilizer application management of wet-rice. In the leaf color scale, leaf colors from 1, which is the palest, to 7, which is the densest, are gradually illustrated, and in the case of reading to the intermediate value of each value, the leaf color can be measured in thirteen stages, and thus, it is possible to easily determine the leaf color. Here, in the measurement, attention such as placing the sun on the back is required, and thus, it is difficult to determine the leaf color according to a measurement time zone. In addition, in a crop such as a rice plant, it is known that in a case where a nitrogen content of the crop increases, a green color becomes dense (a chlorophyll content increases), and for example, the leaf color scale can be a material of determining a period or an amount in fertilizer application such that it is preferable to apply a nitrogen-based fertilizer in a case where a green color of a leaf of a rice plant having a predetermined variety in a predetermined period in a predetermined region is paler than a predetermined density.
The fertilizer application management is performed according to a document in which, for example, a timing or the like for applying the fertilizer is described from the color of the leaf of the rice plant. The document is required to be prepared in each of regions where environments such as climate or soil are similar to each other to a certain degree, and for example, is prepared such that the fertilizer application management or the like can be performed from a date (a period) such as a month and a day, and mainly the color on the leaf color scale at the date. As described above, the document which is used, for example, in the fertilizer application management according to the leaf color scale is required to be prepared in a comparatively narrow range such as a prefecture or a municipal unit, and for example, is prepared by a public institution (a prefectural agricultural experiment station) associated with the agriculture or other organizations (an agricultural cooperative, an agricultural corporation, or the like).
In addition, a chlorophyll meter is used in order to digitize the leaf color, in addition to the leaf color scale described above.
The chlorophyll meter obtains the content of the chlorophyll on the basis of the optical properties of the chlorophyll. It is known that the chlorophyll has an absorbance peak in a range of red of visible light of 600 nm to 700 nm, and rarely absorbs infrared light on a wavelength side longer than 700 nm. Therefore, a chlorophyll amount contained in the leaf, which is measured on the basis of a difference between an optical density of transmission light of the range of red and an optical density of transmission light of infrared light by using an LED outputting light in the range of red and an LED outputting infrared light, is calculated.
As with the measurement result of the leaf color scale described above, the measurement result of the chlorophyll meter, far example, can also be used for determining whether or not the amount of applied fertilizer of the nitrogen fertilizer is suitable, and the measurement result can be used as an index of the fertilizer application management. Furthermore, in the chlorophyll meter, a soil & plant analyzer development (SPAD) value, which becomes an index of the content of the chlorophyll, is output, the SPAD value is associated with a color scale value as the measurement result of the leaf color scale, and thus, the SPAD value and the color scale value can be converted into each other. Accordingly, in both of the SPAD value and the color scale value, it is possible to perform the fertilizer application management by using the document described above relevant to the cultivation of the same rice plant. In the following description, the SPAD value of the chlorophyll meter and the color scale value of the leaf color scale will be referred to as a leaf color value. That is, in the following description, the leaf color value includes the SPAD value and the color scale value.
The object of the chlorophyll meter described above is to measure the content of the chlorophyll and to measure a nitrogen amount of the crop on the basis of the content of the chlorophyll, and a spectroscopic analyzer is also proposed in which nitrogen is measured by optical measurement of the crop (for example, refer to Patent Literature 1). In this case, it is possible to directly measure the nitrogen content and thus, it is possible to use the spectroscopic analyzer in the fertilizer application management.